1. Field of the Invention
The present invention relates generally to inorganic membranes that are permeable to small gas molecules. More particularly, the present invention relates to permeable membranes deposited on porous substrates, having a graded intermediate layer, that exhibit both a high hydrogen permeance and a high hydrogen permselectivity.
2. Description of the Related Art
Permeable materials are those through which gases or liquids may pass. Membranes are one type of permeable material and are composed of thin sheets of natural or synthetic material. Frequently, membranes exhibit different permeances—i.e., permeation rates—for different chemical species. In this regard, permselectivity is the preferred permeation of one chemical species through a membrane with respect to another chemical species. Permselectivity of the desired permeate with respect to another chemical species is calculated as the ratio of the permeance of the desired permeate to the permeance of the other chemical species.
Permselective membranes are promising in a variety of applications including gas separation, electrodialysis, metal recovery, pervaporation and battery separators. Recently, interest has developed in using permselective membranes in so-called membrane reactors, which allow the simultaneous production and selective removal of products. One regime in which permselective membranes are particularly promising is that of equilibrium-limited reactions. In such reactions, yields are reduced by reaction reversibility. Preferential removal of one or more of the reaction products effectively shifts the equilibrium—or, stated differently, decreases the rate of the reverse reaction—thereby overcoming thermodynamic limitations.
One example of an equilibrium limited reaction is the methane dry-reforming reaction [1]:CH4+CO22CO+2H2 (ΔH°298=247 kJ·mole−1)  [1]This reaction provides a pathway to convert carbon dioxide, a problematic greenhouse gas, and methane, a plentiful natural resource, into synthesis gas—i.e., a mixture of hydrogen and carbon monoxide. Synthesis gas is an industrially important feedstock that is used in the preparation of ethylene glycol, acetic acid, ethylene, fuels and several other commercially important chemicals. Unfortunately, the conversion of methane and carbon dioxide to synthesis gas is limited by the reversibility of the reaction—i.e., the ability of hydrogen and carbon monoxide to regenerate methane and carbon dioxide. The yield can be improved, however, by selectively removing one or both of the products as they are formed. Doing so mitigates the extent of the reverse reaction.
Other examples of equilibrium-limited reactions that produce hydrogen gas are the decomposition of hydrogen sulfide [2] and ammonia [3]:H2SS(s)+H2  [2]2NH3N2+3H2  [3]Hydrogen sulfide and ammonia are frequent and undesirable byproducts of numerous chemical reactions. Thus, reactions [2] and [3] offer an abatement technique for reducing the levels of these compounds. Like the methane dry-reforming reaction, the products of these reactions can be favored by removing hydrogen as it is produced. In short, hydrogen permselective membranes offer the potential to overcome several equilibrium-limited reactions in commercially useful ways.
Conventional hydrogen permselective membranes have typically been prepared on porous Vycor™ glass or ceramic supports by sol-gel or chemical vapor deposition (CVD) methods. Generally, a thin silica membrane can be directly coated or deposited on mesoporous supports such as Vycor™ glass with 4 nm pore size, but cannot be placed directly on macroporous supports with pore sizes substantially larger than 50 nm. Hwang et al. attempted chemical vapor deposition (CVD) of tetraethylorthosilicate (TEOS) on a porous alumina tube with pore size of 100 nm and obtained only a selectivity of 5.2 for the separation of H2 from N2 at 873 K after 32 hours of deposition (G-J. Hwang, et al., J. Membr. Sci. 162 (1999) 83). Such low selectivities are indicative of the presence of large pore defects.
Coating macroporous supports using an intermediate mesoporous gamma-alumina sol layer prior to the deposition of a silica membrane has been attempted to overcome this problem of large pore defects. However, the quality of the sol layer is limited by, among other things, the size distribution of the sol particles. On the one hand, as depicted in FIG. 1a), when a dipping solution is used consisting of sol particles that are large compared with the pore size of the supports, the particles do not provide additional restrictive passages for controlling selectivity. Additionally, they do not cover the surface uniformly and can leave patches of exposed, untreated surface. On the other hand, as depicted in FIG. 1b), if a dilute dipping solution is used consisting of sol particles that are small compared with the pore size of the supports, these small sol particles do not easily form “bridges” over some of the large features and extra large pores of the supports because of infiltration during dip-coating. Even if such “bridges” are formed, they are not strong and are easily broken or cracked. This problem becomes increasingly more serious for supports with broader pore size distributions.
Previous work in the literature describes a method of depositing a gamma-alumina layer on a support, for example, (R. J. R. Uhlhorn, et al. J. Mater. Sci. 27 (1992) 527). In this work the dipping-calcining procedure is repeated at least 2–3 times by using a concentrated dipping solution with a boehmite sol concentration of 0.5–1.0 M to obtain a thick, defect-free gamma-alumina layer. The layer thickness of a gamma-alumina supported layer, made with 0.6 M dipping solution containing PVA, was typically 5–6 μm after three subsequent dipping steps with 3 second dipping time. Thinner layers are preferred because thicker layers decrease permeability for the desired permeate.
Although Uhlhorn, et al's method has been used in the past, not much attention has been placed on the physical characteristics of these intermediate mesoporous gamma-alumina sol layers. An ideal intermediate layer would be thin, continuous, defect-free and exhibit a high permeability for the desired permeate.